The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Shock absorbers are typically used in conjunction with automotive suspension systems or other suspension systems to absorb unwanted vibrations that occur during movement of the suspension system. In order to absorb these unwanted vibrations, automotive shock absorbers are generally connected between the sprung (body) and the unsprung (suspension/drivetrain) masses of the vehicle.
In typical shock absorbers, a piston is located within a fluid chamber defined by an outer tube and is connected to the sprung mass of the vehicle through a piston rod. The outer tube is connected to the unsprung mass of the vehicle. The piston divides the fluid chamber of the outer tube into an upper working chamber and a lower working chamber. The piston includes compression valving that limits the flow of hydraulic fluid from the lower working chamber to the upper working chamber during a compression stroke. The piston also includes rebound valving that limits the flow of hydraulic fluid from the upper working chamber to the lower working chamber during a rebound or extension stroke. Because the compression valving and the rebound valving have the ability to limit the flow of hydraulic fluid, the shock absorber is able to produce a damping force that counteracts oscillations/vibrations, which would otherwise be transmitted from the unsprung mass to the sprung mass.
By controlling the fluid flow between the two working chambers, a pressure drop is built up between the two working chambers and this contributes to the damping forces of the shock absorber. The compression and rebound valving and the check valve assemblies can be used to tune the damping forces to control ride and handling as well as noise, vibration, and harshness.
Typical passive shock absorbers provide the same magnitude of damping force regardless of the frequency of the input. For a given input velocity, the damping force generated by a conventional passive shock absorber remains the same regardless of the frequency of the input. Typically, the primary ride frequency of a passenger vehicle is in the range of 1 to 2 Hertz. When a vehicle goes over a road surface with a lower frequency input, a higher amount of damping is preferred to manage the road inputs. During handling events (where directional stability is critical), a higher amount of damping is also preferred. For example, the vehicle may be subjected to body roll during handling events. The frequency of body roll in a typical passenger vehicle commonly ranges from 2 to 4 Hertz depending on the roll-stiffness and the height of the center of gravity of the vehicle. When the damper system experiences larger excitation forces, higher damping forces are required. When conventional passive shock absorbers are used, the higher damping forces result in more harshness and a decrease in ride quality.
Active shock absorbers change the damping of the shock absorber in real-time to address different vehicle suspension inputs. In active shock absorbers, hydraulic, pneumatic, or electro-magnetic actuators are used to apply an active force to the piston rod that is independent of the damping forces generated by the compression and rebound valving.
Unlike passive shock absorbers, active shock absorbers can generate damping forces independently of the velocity of the piston rod inputs. As a result, large excitation forces do not require more hydraulic damping from the shock absorber and therefore do not introduce increased harshness. This is a major advantage of active shock absorbers because it resolves the trade-off in hydraulic damper systems between primary body control (which requires large damping forces) and secondary comfort (which requires low damping forces).
The actuator(s) in typical active shock absorbers are placed in a co-axial arrangement with the damper. In co-axial actuator/damper arrangements, the damper must be designed to accommodate the actuator components. Accordingly, these designs are often expensive to manufacture and there are typically limits on the size of the actuator and the damper due to limited packaging space and their co-axial arrangement.